It is generally well known in the aerospace industry that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in aircraft construction. This desire led to the addition of lithium, the lowest density metal element, to aluminum alloys. Aluminum Association alloys, such as 2090 and 2091 contain about 2.0 weight percent lithium, which translates into about a 7 percent weight savings over alloys containing no lithium. Aluminum alloys 2094 and 2095 contain about 1.2 weight percent lithium. Another aluminum alloy, 8090 contains about 2.5 weight percent lithium, which translates into an almost 10 percent weight savings over alloys without lithium.
However, casting of such conventional alloys containing relatively high amounts of lithium is difficult. Furthermore, the combined strength and fracture toughness of such alloys is not optimal. A tradeoff exists with conventional aluminum-lithium alloys in which fracture toughness decreases with increasing strength.
Another important characteristic of aerospace aluminum alloys is fatigue crack growth resistance. For example, in damage tolerant applications in aircraft, increased fatigue crack growth resistance is desirable. Better fatigue crack growth resistance means that cracks will grow slower, thus making airplanes much safer because small cracks can be detected before they achieve critical size for catastrophic propagation. Furthermore, slower crack growth can have an economic benefit due to the fact that longer inspection intervals can be utilized.
A need therefore exists for an aluminum alloy that is useful in aircraft application which has high fracture toughness, high strength and excellent fatigue crack growth resistance.